The last decade has witnessed a decline in the resources of natural fisheries that has now reached a level of global environmental crisis. Thirteen of the worlds seventeen major fisheries are classified by the United Nations Food and Agriculture Organization (F.A.O.) to be in peril or in steep decline. Even in the best case scenario of both maintaining the fishery catch at a record 100 million metric tons and conservative population growth, there will be a shortfall in global seafood supply by the end of this decade for the first time in human history. Furthermore, per capita global seafood consumption is predicted to increase as the worlds population shifts during the next two decades from industrialized countries to todays undeveloped and emerging economies; traditionally large consumers of seafood. The only way to meet the global demand for seafood by the year 2000 is if aquaculture replaces the insufficient natural fisheries catch.
Expansion of the aquaculture industry requires that several significant problems be addressed, and one of the most significant hurdles to establishing and maintaining an economically viable aquaculture operation is the difficulty of supplying nutritionally balanced feeds. Larval fish, bivalves and crustaceans raised in the wild consume a mixed population of feed organisms that collectively provide balanced nutrition. On the other hand, fish larvae, bivalves and crustaceans raised in aquaculture farms can be difficult to rear and require live feeds (algae or algae-fed rotifers and Artemia) for their nutrition. These live feeds are difficult to produce and maintain, require high labor inputs and specialized facilities, and as a result larval feeds constitute a significant cost to the aquaculture industry.
It is important that aquaculture feeds be nutritionally balanced so that the larvae receive proper nutrition. DHA (docosahexaenoic acid) has been identified as an important nutrient that contributes significantly to larval growth and survival (Fulks, W and K L Main (eds). 1991. xe2x80x9cRotifer and Microalgae Culture Systems. Proceedings of a US-Asia Workshop.xe2x80x9d Honolulu, Hi. The Oceanic Institute. Tamaru, CS, C S Lee and H Ako, 1991. xe2x80x9cImproving the larval rearing of striped mullet (Mugil cephalus) by manipulating quantity and quality of the rotifer, Brachionus plicatilis.xe2x80x9d In: W. Fulks and K L Main (eds), 1991). Larvae ultimately acquire these fatty acids from algae, either by directly feeding on algae with high levels of polyunsaturated fatty acids or by feeding on rotifers and Artemia that have been reared on algae high in polyunsaturated fatty acids. Unfortunately, the algae, artemia and rotifers used at aquaculture farms are low in DHA, reducing the survival rates for the larvae below their maximal rate and increasing the cost of the final aquaculture farm product. If sufficient DHA could be provided to the larvae it is expected that the survival rate for larvae would increase, thus reducing the cost of farm-raised seafood.
In one embodiment, this invention provides a particulate material suitable for use as an aquaculture feed. The particulate material has a high proportion of DHA residues in the lipid fraction, which may be up to 35% of the material. Preferably, the material has a mean particle size of from about 5 microns to about 10 microns.
In another embodiment, this invention provides a method for preparing a particulate material suitable for use as an aquaculture feed from microbial biomass, preferably from algal cells having a high content of DHA residues, by obtaining a lipid fraction from the biomass, preferably by solvent extraction of broken cells, followed by separating a fraction containing phospholipids and proteins from the lipid fraction, and removing water from the protein/phospholipid fraction to form a low moisture particulate, preferably by spray-drying the protein/phospholipid fraction.
In still another embodiment, this invention provides a particulate material containing phospholipids having docosahexaenoic acid (DHA) residues, the particulate material being prepared by drying a slurry comprising a polar lipid extract from DHA-containing microbes, and dry, particulate material may be prepared from a slurry which is substantially free of material that did not come from the DHA-containing microbes, particularly by spray drying the slurry. Typically, at least two thirds of the dry matter in the polar lipid extract of DHA-containing microbes is material derived from the microbial cells; preferably less than 25% of the dry matter is non-microbial, more preferably less than 20%, even more preferably 15% or less. Typically, the spray dried particulate has number average particle size between 5 microns and 10 microns.
In yet another embodiment, this invention provides a method for preparing a DHA-containing particulate material comprising (a) lysing DHA-containing microbial cells; (b) extracting lysed cells with solvent; (c) separating a polar lipid fraction from the extract; and (d) drying the polar lipid fraction, with or without addition of other nutrients, to form a particulate material. Preferably, the polar lipid fraction is an aqueous slurry which is dried by spray drying.
In a particular embodiment, this invention provides a method for preparing a DHA-containing particulate material comprising drying a slurry containing polar lipids extracted from dinoflagellates, wherein the dried material is in the form of particles having a mean particle diameter between 5 and 10 microns.
In yet another embodiment, this invention provides an aqueous emulsion or suspension containing phospholipids with DHA residues obtained from a polar lipid extract from DHA-containing microbes. Preferably, at least 10% of the fatty acid residues in lipids of the DHA-containing microbes are DHA residues. More preferably, at least 10% of the fatty acid residues in polar lipids of the DHA-containing microbes are DHA residues.
In still another embodiment, this invention provides a composition comprising a particulate material containing phospholipids with DHA prepared by drying a slurry comprising (a) a polar lipid extract from DHA containing microbes and (b) a meal containing protein and/or carbohydrate. Preferably, the meal comprises microbial cells or cell fragments, which may be cells or cell fragments which have been extracted to remove part of the lipids, or even most of the lipids. Preferred microbial cells or cell fragments are from Chlorella, Crypthecodinium, or a yeast such as Saccharomyces, or a fungus, such as Morteriella, Schizochytrium, or Thraustochytrium.